When producing elastomeric compositions for use in rubber articles, such as tires, power belts, and the like, it is desirable that these elastomeric compositions are easily processable during compounding and have a high molecular weight with a controlled molecular weight distribution, glass transition temperature (Tg) and vinyl content. It is also desirable that reinforcing fillers, such as silica and/or carbon black, be well dispersed throughout the rubber in order to improve various physical properties, such as the compound Mooney viscosity, elastic modulus, tangent delta (tan δ), and the like. Rubber articles, especially tires, produced from vulcanized elastomers exhibiting these improved properties will have reduced hysteresis, better rolling resistance, snow and ice traction, wet traction, and improved fuel economy for vehicles equipped with such tires.
With the increasing use of silica as a reinforcing filler for rubber, filler dispersion in rubber stocks has become a major concern. Because polar silanol groups on the surface of silica particles tend to self-associate, reagglomeration of silica particles can occur after compounding, leading to poor silica dispersion and a high compound viscosity. The strong silica filler network results in a rigid uncured compound that is difficult to process in extrusion and forming operations. Previous attempts at preparing readily processable, vulcanizable silica-filled rubber stocks containing natural rubber or diene polymer and copolymer elastomers have focused on the use, during compounding, of bifunctional silica coupling agents having a moiety (e.g., a silyl group) reactive with the silica surface, and a moiety (e.g., a mercapto, amino, vinyl, epoxy or sulfur group) that binds to the elastomer. Well known examples of such silica coupling agents are mercaptosilanes and bis(trialkoxysilylorgano) polysulfides, such as bis(3-triethoxysilylpropyl)tetrasulfide which is sold commercially as Si69 by Degussa. These bifunctional silica coupling agents offer excellent coupling between rubber and silica, resulting in rubbers having improved wet ice skid resistance, rolling resistance and tread wear.
However, there are disadvantages to the use of bifunctional silica coupling agents. For example, the high chemical reactivity of the —SH functions of the mercaptosilanes with organic polymers can lead to unacceptably high viscosities during processing and to premature curing (scorch). The tendency of a rubber compound to scorch makes compounding and processing more difficult. Mixing and milling must be done more quickly, yet at lower temperatures (e.g., 120° C. to 145° C.), so that the compound will not begin to vulcanize before it is shaped or molded. Rubber compounds employing bis(trialkoxysilylorgano)tetrasulfide silica coupling agents, such as Si69, must be mixed at a temperature below 165° C., if an irreversible thermal degradation of the polysulfane function of the coupling agent and premature curing of the mixture are to be avoided. The upper processing temperature limitations of the bifunctional silica coupling agents result in a marked reduction in the mechanical activity of mixing which is essential for an optimum dispersion of the silica throughout the polymer matrix. Therefore, compared with carbon black-filled compositions, tread compounds having good silica dispersion require a longer mixing time at a lower temperature to achieve improved performance, resulting in decreased production and increased expense. Moreover, both bis(trialkoxysilylorgano) polysulfide and mercaptosilane silica coupling agents are expensive.
Another disadvantage of the use of bis(trialkoxysilylorgano) tetrasulfide and mercaptosilane silica coupling agents is that the upper processing temperature limitations result in a relatively slow rate of the chemical reaction between the alkoxysilyl portion of the silica coupling agents and the silica (the alkoxysilane-silica reaction). Because this reaction results in the release of a substantial amount of alcohol, a slow reaction rate results in the presence of unreacted alkoxysilyl groups in the compounded product that are then available to further react with the silica and moisture during storage, extrusion, tire build, and/or curing, resulting in an undesirable increase in the compound viscosity, and a shorter shelf life. Moreover, the continuing reaction in the compound evolves more alcohol, resulting in porous zones or blisters which can form surface defects in the resulting formed rubber articles and/or can impair the dimensional stability of treads during extrusion and tire building. As a result, a low tread strip drawing speed must be maintained to ensure that the drawn product conforms with specifications, resulting in a further decrease in production and concomitant increase in costs.
To address the expense and other problems related to bifunctional silica coupling agents, recent approaches to improving dispersion of silica in rubber compounds have been directed to reducing or replacing the use of such silica coupling agents by employing silica dispersing aids, such as monofunctional silica shielding agents (e.g., silica hydrophobating agents that chemically react with the surface silanol groups on the silica particles but are not reactive with the elastomer) and agents which physically shield the silanol groups, to prevent reagglomeration (flocculation) of the silica particles after compounding. For example, silica dispersing aids, such as alkyl alkoxysilanes, glycols (e.g., diethylene glycol or polyethylene glycol), fatty acid esters of hydrogenated and non-hydrogenated C5 and C6 sugars (e.g., sorbitan oleates, and the like), polyoxyethylene derivatives of the fatty acid esters, and fillers such as mica, talc, urea, clay, sodium sulfate, and the like, are the subjects of EP 890603 and EP 890606. Such silica dispersing aids can be used to replace all or part of expensive bifunctional silica coupling agents, while improving the processability of silica-filled rubber compounds by reducing the compound viscosity, increasing the scorch time, and reducing silica reagglomeration. To achieve a satisfactory cure of the rubber compound, the use of silica dispersing aids includes employing an increased amount of sulfur in a mixing step when curing agents are added to the composition, to replace sulfur that otherwise would have been supplied by a sulfur-containing silica coupling agent.
An advantage of the use of silica dispersing aids during compounding of elastomers with silica is that, unlike the bifunctional silica coupling agents described above, the dispersing aids do not contain sulfur and, thus, they can be used at high temperature, e.g., about 165° C. to about 200° C., in the absence of curing agents, without increasing the risk of premature curing. At these high temperatures, the reaction between the silica and alkoxysilyl groups of alkyl alkoxysilane silica dispersing aids is accelerated, resulting in an increase in the amount of alcohol evolved and evaporated during compounding, and a decrease in evolution of alcohol from the compound during storage, extrusion, curing and tire build.